Certain sensors utilize bolometers. Bolometers detect changes in a scene's actual or radiometric temperature. The ultimate sensitivity of a bolometer is limited by thermal loading. A bolometer's sensitivity is characteristically given in terms of temperature: noise equivalent delta temperature (NEDT). Neglecting readout circuit's noise, a conventional bolometer's equivalent thermal circuit is shown in FIG. 1. The bolometer's sensing element operates at temperature TD and receives radiation from the scene, at temperature TS, through space/optics that are represented by conductance GR. The bolometer is loaded by two thermal conductances: GD, representing radiative loading through a 4π solid angle; and by G2, representing thermal loading from legs by which the sensing element is suspended (see FIG. 2). The sensing element's heat capacity is CD.
Bolometers operate as thermal equilibrium devices and using heat flow balance equations, a relationship relating small changes in scene temperature (δTS) to changes induced in the bolometer's temperature (δTD) is given by Equation (1) below. Thermal loading by GD and G2 produce attenuations that result in a much smaller temperature change in the bolometer's in response to a change in scene temperature: (δTD)<<(δTS). This attenuation is large for long-wave infrared (LWIR) (about 10−2) and much larger at 95 GHz (about 10−4). Attenuation by thermal loading leads to two deleterious effects: (1) direct reduction in sensitivity, neglecting readout circuits noise, and (2) further sensitivity degradations because with smaller signals, readout circuits noise becomes more prominent. Thermal loading directly reduces NEDT because thermodynamic temperature fluctuations [TS(k/CD)1/2] are not attenuated while the signal is attenuated by GR/(GD+G2): hence NEDT is degraded by [(GD+G2)/GR]1/2. Additionally, noise from the readout circuits further degrades the bolometer's NEDT.
                                          δ            ⁢                                                  ⁢                          T              D                                =                                                    G                R                                                              G                  D                                +                                  G                  2                                                      ⁢                          1                              [                                  1                  +                                      jω                    ⁡                                          (                                                                        C                          D                                                                                                      G                            D                                                    +                                                      G                            2                                                                                              )                                                                      ]                                      ⁢            δ            ⁢                                                  ⁢                          T              S                                      ⁢                                  ⁢                  NEDT          =                                                    [                                                                            G                      D                                        +                                          G                      2                                                                            G                    R                                                  ]                                            1                /                2                                      ⁢                                                            kT                  S                  2                                                  C                  D                                                                                        (        1        )            
With reference now to FIG. 2, shown is a conventional bolometer 10. As shown, conventional bolometers 10 are suspended by two legs 12. A square sensing element 14 (about 30 μm for LWIR) is suspended by the legs 12 which also form bridges between the bolometer 10 and a heat bath (not shown). Each leg 12 incorporates an insulator for mechanical support and a thin conductor for electrical readout of the sensing element 14. Consequently, these legs 12 provide mechanical support, thermal isolation and electrical access for readout. For maximum thermal isolation, each leg 12 is made long with a small cross section. Additionally, each leg 12 is made from materials with the lowest thermal conductivity conductors and insulators. Characteristically, insulators have a much smaller thermal conductivity than conductors. Consequently, the insulators are made about 1 μm thick and used for mechanical support, while the conducting lines are made as thin as possible, about 30 nm.
The thermal conductivity of silicon dioxide at 300K [κSO=1 Watt/M-K] is at least ten times smaller than the thermal conductivity of a conductor used. Nichrome is a conductor with a poor thermal conductivity, equal to κN=12 Watts/M-K at 300K. Stainless Steel is another example of a poor thermal conductivity conductor, with κSS=30 Watts/M-K. For LWIR, with small 30 μm pixels, the thermal conductivity for each leg utilizing silicon dioxide and with minimum geometry is about G2=50 nW/K. This thermal conductivity is much larger than radiative conductivities GD=7.7 nW/K, and GR=0.96 nW/K, resulting in the large degradations in sensitivity. At 95 GHz, the pixel size is larger (about 1 mm) hence with longer bridge legs the thermal conductance of G2 can be made significantly smaller. However, mechanical support requirements limit how much longer these bridge legs 12 can be made. More importantly, the value of GR≈5×10−12 is much smaller than at LWIR resulting in even more signal attenuation. Clearly a different technique is needed to circumvent the thermal loading problem, particularly at 95 GHz, and beyond into the so called Terahertz imaging gap.
A ultra sensitive silicon sensor (USSS) described below provides an improved technique to circumvent the thermal loading problem. However, using conventional microantennas in the USSS approach presents its own problems. First, sensitivity at LWIR only in the center portion of conventional microantennas is grossly insufficient to recover pixel fill factor efficiency. Second, a narrow FOV provided by conventional microantennas further reduces the radiation signal.